Here is something I found in an SMF support forum article on the best performance tweaks
Move your uploaded avatars directory
In the administration panel, go to Attachments and Avatars, and then go to Avatar Settings.
For Upload avatars to... select: Specific directory... For Upload directory: put in the raw, publicly accessible server directory you want them added to ie /var/www/yoursite/docs/forums/avs
So assuming /docs was your webroot, you would put /forums/avs for your upload URL. Don't do what I did and make it a subdirectory of your main avatars directory, SMF still handles it like an attachment directory for some reason so if someone uploads a .png avatar, someone else decides to use it and the first person replaces it with a .jpg avatar, the first will be deleted.
The benefit of this borders on the ridiculous. Every single avatar load is another php call, which loads a session, the user's settings and permissions, the board and its theme. In addition, two additional calls to the database are made to load the avatar itself, and to increment the view counter. So not only are you reducing the processor load on your server, but you are also reducing your database load. If five different avatars are displayed on a page, that's five additional php calls and dozens of database queries getting made.
After you do this access index.php?action=manageattachments;sa=moveAvatars to use the hidden feature to move your avatars over.
This setting is in
CountryPlans Design/Build Forum »
Administration Center »
Attachments and Avatars »
It looks like we have an Avatar directory under the SMF dir. Should we build another folder outside the SMF forum - perhaps avatars in the public section?
PS - Looking at the error logs on Site 5 a lot of our errors are attempting to load avatar images (ie: smf/avatars/dilbert.gif). Those errors are coming in at 5 to 10 per minute. They are mostly image requests. Some are for an "apple-touch-icon.png" which lots of sites apparently get as an error message from page requests from iphones.
PPS - Looking at the file manager the above avatar directory only has old preloaded images from 2004, not the images most members are using as avatars.